As the application of a pouch type lithium ion secondary battery has spread in recent years, the energy density of batteries and the energy storage capacity of a unit cell have gradually increased. Particularly, to meet requirements brought from an increase in the mileage of a vehicle, it has been actively studied in recent years to find a technology that can increase the energy storage capacity of a lithium ion secondary battery for vehicles. However, the conventional pouch type lithium ion secondary battery is problematic in that the sheath of the battery has a low physical strength, and the battery is not provided with a cell level safety device (current interruptive device (CID), Positive Temperature Coefficient (PTC), fuse, etc.). Therefore, it is difficult to realize desired safety of a battery having a high energy storage capacity. Particularly, when a vehicle battery is overcharged, gas is generated in the battery, and the temperature inside the battery increases, so thermal runaway may be induced in the material inside the battery and may result in a fire. Further, when an abnormal reaction is generated in even just one unit cell when driving the vehicle, an abnormal voltage may be easily induced in a battery pack, thereby reducing the driving safety of the vehicle.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.